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Chinese Physics Letters

Pump Intensity Dependence of Two-Beam Coupling in Doped Lithium Niobate Crystals

Nouel Y. Kamber, Jingjun Xu, Sonia M. Mikha, Song Feng, Guoquan Zhang, Zhang Xin-Zheng, Liu Si-Min, Zhang Guang-Yin

Published Jun 1, 2000• 0 citations
Publisher

Abstract

We demonstrated experimentally the dependence of two-beam coupling on the incident pump intensity in our samples of doped LiNbO 3 crystals. Our results show that there is an optimum pump intensity for the signal beam amplification, which can be easily controlled by doping the LiNbO 3 crystal with suitable concentrations of Fe and damage-resistant dopants such as Mg, In, and Zn.

DOI
10.1088/0256-307x/17/6/016

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Optics Letters•Sep 15, 1996•34 citations
We demonstrated experimentally the dependence of two-beam coupling on the incident pump intensity in our samples of doped LiNbO 3 crystals. Our results show that there is an optimum pump intensity for the signal beam amplification, which can be easily controlled by doping the LiNbO 3 crystal with suitable concentrations of Fe and damage-resistant dopants such as Mg, In, and Zn.
The light amplification mechanism of fanning noise in doped lithium niobate crystals was analyzed and the light amplification competition between the signal beam and the fanning noise was discussed. Results show that, due to the light amplification competition between the signal beam and the fanning noise, there exists an optimum photovoltaic field in doped lithium niobate crystals and an optimum pump intensity for the light amplification of signal beam. With such optimum conditions, the signal beam can be amplified maximally while the fanning noise is very weak.
The buildup and the decay of photorefractive two-beam coupling are experimentally demonstrated to be strongly dependent on the intensity ratio of the two beams (signal-to-pump ratio). The experiment, done with a BaTiO3 crystal, shows good agreement with the theory of two-beam coupling in the depleted-pump regime. The deviation of the response behavior from the small-signal regime is seen for ratios as small as ∼10−5. These results are significant for applications of two-beam coupling as gain media, especially for the elimination of amplified noise (fanning).
Although noiseless optical amplification has been demonstrated in bulk KTP crystals, and is well understood, there have been no observations of noiseless optical amplification in bulk quasi-phase-matched (QPM) systems. That such an amplifier is possible was predicted recently and indeed squeezing has already been observed in QPM waveguides (using KTP and LiNbO3). QPM materials are attractive for use in quantum-optical devices because they give access to certain (large) components of the nonlinear susceptibility tensor that cannot be used in conventional birefringent phase-matching schemes. They also extend the range of materials accessible for use in parametric amplifiers to include some low-birefringence crystals exhibiting large nonlinearities. Even with the technological advantages offered by certain QPM crystals, efforts to find practical applications for optical parametric amplifiers are hampered by the need for enormous pump-field strengths. The obvious solution to the problem of the pump-field requirement is to confine the incident energy to a narrow interaction region thereby increasing the energy density. However, this approach can only be realised at the expense of sacrificing the homogeneity of the parametric interaction between pump and signal waves. This in turn causes vacuum field fluctuations from parasitic modes to be mixed into the signal mode thereby degrading the signal to noise ratio and negating the advantage of using an optical parametric amplifier. In the experiments presented here, optical signals have been amplified with up to 9dB of gain, yet with a noise figure (NF) of only 1.09, as compared to the minimum possible NF of a classical amplifier with the same gain which would equal 1.86. Furthermore, the interaction is achieved with tightly focused incident fields in a sample of bulk QPM lithium niobate. The entire interaction region is only 200µm thick and 2.8mm long. This performance was achieved with the aid of a theoretical model m which the effect of wavefront curvature and mode distortion are treated within a quantised mode framework using a spatial mode basis. Such a simulation permits the fundamental origin of the excess quantum noise to be easily traced to zero-point fluctuations of high-order spatial modes which couple to the signal mode via the parametric interaction. Furthermore the simulation enabled an experimental geometry to be selected in which an optimal compromise is made between wavefront matching bump beam smaller than signal) and pump-homogeneity (pump larger than signal), so that the overall noise figure remains well into the quantum-performance region.
An optical parametric amplifier (OPA) has been demonstrated in bulk, periodically poled lithium niobate and is shown to operate with a noise figure well below the classical limit. In contrast to conventional OPA's, this device uses quasi-phase matching to provide the coupling between the pump and the signal. Comparison of the measured performance with that of a theoretical model reveals that the main intrinsic contribution to the output noise is due to spatial and temporal mode mixing, which arises as a consequence of tight focusing of the incident beams. Factors that affect the performance of this amplifier are identified theoretically and their relative importance investigated for both amplification and squeezing.